Dynamical heterogeneity, in which transitory local fluctuations occur in the conformation and dynamics of constituent particles, is widely hypothesized to be essential to the evolution of supercooled liquids into the structural glass state. Yet its microscopic spatiotemporal phenomenology is challenging to detect directly in molecular glass forming liquids. Because recent theoretical advances predict that corresponding dynamical heterogeneity could occur in supercooled magnetic monopole fluids (Proc. Nat. Acad. Sci. 112, 8549 (2015)), we searched for such phenomena in Dy
2
Ti
2
O
7
. By measuring its microsecond-resolved spontaneous magnetization fluctuations
M
(
t
,
T
)
we detected a sharp bifurcation in monopole noise characteristics below
T
≈
1,500
mK
, with the appearance of powerful spontaneous monopole current bursts. This intense dynamics emerges upon entering the supercooled monopole fluid regime, reaches maximum strength near
T
≈
750
mK
and then collapses along with coincident loss of ergodicity approaching
T
g
≈
250
mK
. Moreover, when the four-point dynamical susceptibility
χ
4
(
τ
,
T
)
is determined directly from temperature dependence of correlations in
M
(
t
,
T
)
, it evolves as predicted when dynamical heterogeneity is present, revealing its simultaneously and rapidly escalating length and time scales,
ξ
(
T
)
and
τ
4
(
T
)
. This overall phenomenology greatly expands our empirical knowledge of supercooled monopole fluids and, more generally, demonstrates techniques for detection of the time sequence, magnitude, statistics, and correlations of dynamical heterogeneity, access to which may greatly accelerate fundamental vitrification studies.
